Gull-billed Terns (Sterna nilotica) (hereafter, GBT) have experienced population
declines in coastal Virginia and elsewhere along the Atlantic Coast, with numbers in
Virginia dropping from 2000+ breeding individuals in the mid-1970s to between 600-800 the
past three years (1993-96). Low breeding productivity has been suspected over the past
decade (B. Truitt, pers. obs.). GBTs nest on two types of habitats along coasts - on
barrier island sandy beaches, often associated with other terns and Black Skimmers (Rynchops
niger) , and on shellpiles along the fringes of saltmarsh islands in coastal lagoons.
Populations using these two habitats are subjected to quite different selective pressures
as a result of physical and biological differences. Storm events (average of 38/yr in
coastal Virginia) are much more dramatic in changing barrier island profiles than
shellpiles. Also, predators, including gulls , raccoons and foxes, are more abundant (but
often unpredictable in location) on barrier islands than on shellpile habitats. On the
other hand, barrier islands often attract large numbers of other sand-nesting species
which may be rare or absent on the size-limited shellpiles.

Here we report on one landscape question concerning the role of habitat stability on
colony site fidelity (colony turnover) : how do physical (e.g. storms) vs.
biological factors (predation, social attraction) influence the probability of colony
sites being reoccupied between years?

Two competing hypotheses are tested:

1. Physical and/or predator disturbances on barrier islands result in higher colony
site turnover between years than on shellpile habitats in marshes.

2. The attraction of larger numbers of both conspecifics and related species on barrier
islands (relative to shellpiles) results in lower colony site turnover on barrier islands
vs. shellpiles.

We investigated the nesting ecology of this species by taking a two-tier approach: the
first was an extensive one in which we estimated the nesting populations of all GBT
colonies at all locations in coastal Virginia from 1993 to 1996; the second was an
intensive one (1994-96) in which from four to six colonies were monitored 1-3 times weekly
for nest success. At each site, a sample of from 8 to 50 nests was marked and followed
during the nesting period. Sites were chosen in both habitats, barrier island and
shellpile; however, the number of barrier island sites monitored was limited to only 1-3
whereas 3-4 sites on shellpiles were monitored.

Colony site turnover (in fact, the percentage of colonies present in only one of any
two years compared) was calculated after Erwin et al. 1981 (Auk 98). Turnover was compared
for each of the consecutive years from 1993 to 1996 separately for barrier vs. shellpile
habitats. A site was considered occupied by a colony if at least 5 nests were
found. Also, a colony site was considered distinct if it was located a minimum of 200 m
from another site.

Colony site turnover was high (>30%) in four of the six yearly comparisons made
(Table 1). Of the total of 25 sites used over the four-year period, only three were used
all four years. Turnover was higher on average on shellpile sites than on barrier islands
(Table 1). Most colony sites (16 of 25) were used only one or two of the four years (Fig.
1).

Related to colony site shifts, we found flooding to be more frequent on shellpiles than
on the barrier islands examined (Table 2). However, birds tended to renest in the same
year at the same sites even after a complete washover.

Colony sizes of GBTs on barrier islands tended to be larger than those on shellpiles,
although the variances and limited samples resulted in no significant differences
(P>0.05) (Table 3).

Table 1. Colony Site Turnover by Gull-billed
Terns (1993-1996)

Comparison1

Habitat

1993-94

1994-95

1995-96

Mean

Barrier islands

0.162 (6/4)

0.41 (4/7)

0.32 (7/5)

0.30

Shellpiles

0.71 (8/6)

0.72 (6/9)

0.22 (9/9)

0.55

1Turnover, t, expressed as percent colonies used only
1 of 2 years (Erwin et al. 1981 Auk). N sites in the 2 years (in parentheses)

Because the largest GBT colonies were associated with some of the largest Common
Tern-Black Skimmer colonies in the state, we suggest that the second hypothesis is more
likely supported than the first. Further, two of the three colony sites occupied all four
years of the study were on these barrier islands. Although flooding probabilities were
higher on shellpiles, birds tended to renest later in the season at the same sites rather
than relocating. If flooding were the critical factor, one would expect large-scale shifts
from shellpiles to barrier islands following flooding. Others (Burger and Gochfeld 1991)
have suggested that, for skimmers, flooding does not often lead to colony relocation.

Previous nesting success is often the best indicator of habitat quality for a species.
Our preliminary data suggest very poor success in all three years in both habitats. Often,
less than half the nests that hatch young produce at least one chick. We do not feel this
factor would lead to a habitat difference in fidelity.

Colony site turnover is higher in GBTs than for other related species. Further work is
needed on the species in other parts of the coastal U.S.

GBT colonies nesting on shellpiles are frequently flooded during 'new moon' high tides.
Colonies were flooded at least once each year during the three years of our study. To
reduce flooding, we recommend augmentation using shells or sand to elevate the shellpiles
a minimum of 15 cm. This procedure could be effective in areas as small as 250 square
meters. In Virginia, at present, clam and oyster shells are used only for oyster reef
restoration.

GBT colonies, especially those nesting on barrier islands, are also subject to
disturbance by recreational boaters and beachcombers. More effective protection of
critical nesting areas is required. Warning signs should be installed around colonies by
10 May of each year and left in place through the end of the nesting season (August or
September). Areas with very high densities of recreationists may require the use of
wardens to patrol the islands and prevent colony disturbances.

Predator populations have increased on the Virginia barrier islands over the past 25
years (B. Truitt, pers. obs.), causing many GBT colonies to relocate to other
predator-free sites. Islands that currently maintain viable predator populations have no
GBT colonies nesting on them. Predators also appear to be expanding into nearby areas that
do contain GBT colonies. Removing predators (i.e. foxes, raccoons, and in some instances
gulls) from the islands could restore valuable nesting habitat.

1. Gull-billed Terns in coastal Virginia exhibit colony site changes (between-years)
that are similar to (barriers) or higher than (marsh shellpiles) those reported for
similar species in the region. Only 3 of 25 colony sites were occupied in all 4 years of
the study.